Interruptions in the 28S rRNA coding regions of most rDNA repeats in Drosophila species have features that relate them to transposable elements. One is the fact that there are sequences otuside of nucleolus organizers that are homologoos with portions of the interruptions. There are also degrees of homology among rDNA interruptions in distantly related genera of Diptera. At present, we have recombiannt DNA libraries of the genomes of D. virilis, D. melanogaster, and the flesh fly Sarcophaga bullata; we are selecting cloned segments for hybridization and DNA sequence analyses that will reveal patterns of conservations and divergence of rDNA interruptions and related sequences. Knowledge of the sites and the nature of conserved sequences and other points should provide some understanding of the origin and establishment of rDNA interruptions and their relatives. The rDNA interruptions are essentially not transcribed, and their role is obscure. A consequence of their presence in rDNA is that repeats which contain them are not expressed even in part, we plan to learn the contexts of various non-rDNA relatives of the rDNA interruptions since there may be comparable situations outside of rDNA. We will attempt to learn the basis of transcriptional inactivity of interrupted repeats in two ways, both involving in vitro reconstruction of rDNA plasmids to contain components of intact and interrupted repats: In one, we will use an in vtrio Drosophila RNA polymerase I transcription system we are developint to attempt to pinpoint the defect in interrupted rDNA. In vivo integrity of chromatin may be involved, so the other approach will be to inject early embryos with recombinant rDNA and look for expression later in development; since D. melanogaster and D virilis rDNAs have distinguishable external transcribed spacers, interspecific recombinants will be used. In Drosophila, most non-rDNA homologues of components of rDNA interruptions are in chromocentric hterochromatin. Advantages will be taken of this in determining in clones the DNA composition of forms of heterochromatin other than that which contains satelllite DNA, and to learn somethin of the relationshp of this material to the rest of a genome. This will certainly include the isolation and examination of authentic transposable elements and other features of dipteran genomes, information on the properties and interspecific relatedness of families of these elements and other components will accrue.